3GPP Long Term Evolution (LTE) is a standard for mobile device network technology. LTE is a set of enhancements to the Universal Mobile Telecommunications System (UMTS), and is a technology for realizing high-speed packet-based communication that can reach high data rates on both downlink and uplink channels.
The current Radio Link Control (RLC) protocol in LTE and HSPA defines a polling bit, which triggers a status message transmission in the receiver peer, unless it has already been triggered. Typically, the status message will be transmitted back to the transmitter peer immediately, or as soon as possible. However, if either a reordering timer or a status prohibit timer is running, the transmission of the status may be delayed. The reordering timer may be started, for instance, whenever a gap in sequence numbers is detected and its expiration may trigger a status report to be sent from the receiver peer to the transmitter peer. The status prohibit timer is started whenever a status is sent.
The 3GPP specification provides a description of polling in an Acknowledged Mode (AM) receiver. For instance, 3GPP TS 36.322, Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Link Control (RLC) protocol specification, version 11.0.0, sub-clause 5.2 specifies the Automatic Repeat-reQuest (ARQ) procedures used for AM data transfer. As described in the standard's specification, the AM RLC entity polls its peer AM RLC entity in order to trigger STATUS reporting at the peer. Upon assembly of a new Acknowledged Mode Data (AMD) Protocol Data Unit (PDU), the transmitting side of an AM RLC will increment the value of “PDU_WITHOUT_POLL” by one, and increment the value of “BYTE_WITHOUT_POLL” by the number of contained bytes. If either PDU_WITHOUT_POLL is greater than or equal to a value, pollPDU, or the counter “BYTE_WITHOUT_POLL” is greater than or equal to a value, “pollByte,” a poll will be triggered. Further, if both the transmission buffer and the retransmission buffer are emptied, or if no new RLC data PDU can be transmitted, e.g., due to window stalling, or a counter t-PollRetransmit expiring, while conditions for “Last Data Poll” persist, a poll will be triggered. According to the standard, the RLC AM entity maintains a variable RETX_COUNT to count the number of retransmissions of an AMD PDU. Presently, there is one RETX_COUNT counter per PDU that is retransmitted.
The pollPDU and pollByte are optional thresholds which are configured by the radio resource layer in the evolved nodeB (eNodeB), using Radio Resource Control (RRC) signalling to configure the transmitter in the user equipment (UE). These thresholds are set per each RLC AM entity. The transmitting side of an RLC AM entity will start or restart t-PollRetransmit each time it delivers a PDU including a poll to a lower layer, i.e., the timer is (re)started on the transmission occasion. At the same time, the entity remembers the sequence number (SN) of the PDU for which the poll was set, as outlined in 3GPP TS 36.322, subclause 5.2.2.1 (“set POLL_SN to VT(S)—1”). The t-PollRetransmit timer is stopped upon reception of a STATUS report comprising either a positive or a negative acknowledgement for the RLC data PDU with sequence number equal to POLL_SN. A positive acknowledgement (confirmation of successful reception) is used to advance the transmitter window and to send an indication of successful delivery to higher layers. A negative acknowledgement (notification of reception failure) will make the transmitter retransmit, thereby restarting the timer as described above. This occurs only if the SN of the corresponding RLC PDU is within the transmitter window. For instance, it could have been already acknowledged and used to advance the transmitter window. Also, the above only occurs if the RETX_COUNT has not exceeded the maxRetxThreshold that corresponds to the PDU.
In Section 5.2.3, 3GPP TS 36.322 also provides a description of status reporting. The acknowledgements of received PDUs or byte segments of PDUs are sent in Status Reports. There are two types of triggers for such reporting.                Transmitter triggers: The receiver may be explicitly polled by the transmitter as described above. The transmitter's aim is to progressively report successful delivery to higher layers and advance its transmission window. It cannot do that without continuous feedback from the receiver (positive acknowledgements).        Receiver triggers: The receiver itself may trigger status reporting when detecting a reception failure, appearing as a gap in the SNs of successfully received PDUs. The receiver's aim is to progressively advance its receiving window by reassembling SDUs for further delivery through higher layers. The gap effectively hinders it from delivering out-of-order SDUs. The gap cannot be filled unless the transmitter performs retransmission of the missing parts. Thus, the receiver supervises the occurrence of one or more gaps with a timer, t-Reordering, which is set proportionately to avoid short-cutting retransmission schemes of lower layers. Status reporting will be triggered if the gap still occurs at the time of t-Reordering's expiration. The timer is stopped when gap(s) cease to exist.        
The RLC receiver sends a STATUS PDU if each of the following are fulfilled:                status reporting is triggered (as described above);        it is allowed to send status (t-StatusProhibit is not running); and        a transmission opportunity is provided.The receiving side of an RLC AM entity will start or restart the t-StatusProhibit timer each time it delivers a PDU, including status to lower layer. In other words, the timer is (re)started with the transmission occasion. Values for t-StatusProhibt are enumerated in 3GPP 36.331.        
3GPP 36.322, Section 6.2.2.11 describes a Polling bit (P) field. Presently, the P field indicates whether or not the transmitting side of an AM RLC entity requests a STATUS report from its peer AM RLC entity. Specifically, a Status Report is requested if P=1. The poll is sent by sending a message with the P bit set in the RLC PDU or RESEG PDU header based on triggers described above. The poll is usually sent along with the transmission of a new data PDU. The poll may also be sent along with a retransmission of a previously sent AMD PDU or RESEG PDU if the “Last Data Poll” trigger is activated, i.e., no new data can be sent.